The invention relates to a rotatable injection timing device for internal combustion engines and more particularly diesel engines provided with at least one servo piston, said device being operable by the pressure of a hydraulic medium, controlled at least in dependence on the speed, against a resetting force, said at least one servo piston adapted to vary the mutual rotary position of coaxially arranged input and output shaft members by means of an adjusting gear mechanism. Such an injection timing device has been known (Swiss Pat. No. 331,948) wherein the relative rotary position of the output shaft with respect to the input shaft can be varied by servo-actuated servo pistons accommodated within the timing device and arranged to be effective radially toward the outside. One of the servo pistons contains a control slide valve operated by centrifugal force and is effective against the bias of a resetting spring, this slide control valve being arranged to control the position of the servo piston. The transmission of the adjusting movements takes place via rocker arms resting on adjusting blocks connected to the output shaft, and the force-derived connection between the rocker arms and adjusting blocks is maintained by helical compression springs, in addition to the driving torque which also serves for this purpose. In this timing device, the instant of injection can be controlled exclusively in dependence on the speed, and the torque fluctuations caused by the cam drive mechanism of the injection pump can be damped only inadequately by the helical compression springs. These torque fluctuations lead, due to the direct path transmission of the adjusting gear system which includes the rocker arms, to rotational torsional vibrations and thus to short-term angular changes between the input and output sections; this results, in addition to creating a loud drive noise, also in an unintended, extensive load dependability of the adjusting operation. The above-described disadvantages furthermore lead to a rapid wear of the transmission parts and thus to a change in the injection timing, which cannot be tolerated, especially in case of modern engines due to the increased demands regarding the exhaust gas quality.
It is true that injection timing devices operated by centrifugal force are known from German Pat. No. 1,022,419 and British Pat. No. 816,498 wherein the flyweights engage the adjusting eccentrics of eccentric pairs constituted by adjusting and compensating eccentrics and serving as the adjusting gear system. On account of the self-locking action inherent in the construction of these injection timing devices, the latter show a low dependence on the load in case of load surges and consequently also exhibit a low wear sensitivity; besides, the noise produced by this system is low. However, these timing devices also can only be controlled in dependence on the speed, and they are limited in their operating capacity by the size of the flyweights and of the space available for installation of the resetting springs.